[antlr-interest] What does empty subrule mean?

[Arto Viitanen]

I had following in my antlr 3 .g file:

statement
   :
    | (ID '(' argumentList ')' ';' )=> procedureCall
    |  ('break'  ) => 'break' ';' -> ^(STAT 'break')
    | ('continue' ) => 'continue' ';' -> ^(STAT 'continue')
    ....

I had had procedure calls that needed at least one argument. Then I
noticed that it should be possible to have
procedure calls without any arguments. So I tried both

(ID ';') => procedureCall

and making argumentList something like argument* (ok, with commas). So I
removed the first line, but left
the | mark. This made the antlr tool generate lots of warnings, but no
error. Of course, it took sometime, before
I noticed the extra | ...

So, why is

rule : | subrule | subrule ...

allowed and what does it mean?

--
Arto Viitanen